This invention relates to water-absorbent carboxyl-containing polymers having a low residual monomer content and a process for their preparation.
Water-absorbent polymers, also referred to as superabsorbent polymers or aqueous fluid absorbent polymers, are primarily used in personal care products which absorb body fluids, for example, baby diapers, adult incontinence products and feminine hygiene products. In such applications, superabsorbent polymer particles are incorporated into absorbent structures that contain synthetic and/or natural fiber or paper based, woven and nonwoven structures, or toughened masses of fibers, such as fluff pads. The materials used in such structures can quickly absorb aqueous fluids and distribute them throughout the whole absorbent structure. The structures, in the absence of superabsorbent polymers, have limited absorption capacity, are bulky due to the large amount of material needed to provide acceptable absorption capacity, and do not retain fluid under pressure. A means for improving the absorbency and fluid retention characteristics of such absorbent structures is to incorporate superabsorbent polymer particles that imbibe fluids to form a swollen hydrogel material.
The superabsorbent polymer particles quickly absorb fluids and retain such fluids to prevent leakage and give the absorbent structure a “dry feel” even when wetted. See U.S. Pat. No. 4,610,678 for examples of such polymers. See also U.S. Pat. No. 4,654,039 and Re. No. 32,649, which disclose a process for the preparation of superabsorbent polymers and the use of known crosslinking agents for such polymers, and also U.S. Pat. Nos. 4,295,987 and 4,303,771. A variation of the basic process is taught in GB Patent 2,119,384, which discloses a post polymerization surface crosslinking process in which the previously polymerized absorbent polymer powder is mixed with crosslinkers, preferably polyalcohols, a solvent and water, to coat the polymer surface and is heated to temperatures in the range of 90 to 300° C. to crosslink the surface. U.S. Pat. No. 5,506,324 discloses superabsorbent polymer particles comprising polymers containing carboxyl moieties which are crosslinked using C2-10 polyhydric hydrocarbons which are ethoxylated with from 2 to 8 ethylene oxide units per hydroxyl moiety of the polyhydric hydrocarbon wherein the hydroxyl moiety at the end of each ethylene oxide chain is esterified with a C2-10 unsaturated carboxylic acid or ester thereof. In a preferred embodiment, the superabsorbent polymer particles are subjected to a heat-treatment process after drying and sizing the particles.
A basic problem with commercially available water-absorbent polymer has been the presence of residual monomers. A water-absorbent polymer product with reduced residual monomer content is highly desired. Various methods to lower the residual monomer content are known in the art.
European Patent Publication 505 163 relates to a method for reducing residual (meth)acrylic acid present in poly(acrylic acid) water-absorbent gel polymers which comprises treating these polymers with a combination of a surfactant having a certain HLB and a vinylic addition compound that can react with a vinylic double bond. Examples of the vinylic addition compound include sulfites and bisulfites. The surfactant and the vinylic addition compound may be used in admixture with oxidizing anions, such as peroxodisulfate and peroxide. An aqueous solution of the additives is mixed with the water-absorbent polymer in the form of swollen gels or beads or dry polymer. The presence of surfactants is believed to negatively affect the liquid distribution when the polymer is wetted. In addition, vinyl addition compounds like sulfites and bisulfites generally cause offensive odor problems during processing.
U.S. Pat. No. 5,629,377 discloses a process for preparing water-absorbent polymer particles comprising polymerizing unsaturated carboxyl-containing monomers in the presence of a chlorine- or bromine-containing oxidizing agent to form a hydrogel which is then heated at a temperature of from 170° C. to 250° C., preferably of from 210° C. to 235° C. Alternatively, the chlorine- or bromine-containing oxidizing agent may be added to the polymerized hydrogel. The method is effective for improving absorbency, for example, centrifuge capacity and absorbency under load (AUL), while keeping the amount of residual monomers at an acceptable level. However, the high heat treat temperature needed to activate the chlorine- or bromine-containing oxidizing agent is detrimental for various reasons, including high energy usage and loss of moisture.
U.S. Pat. No. 4,659,793 teaches that very small amounts of certain metal ions (Zn2+, Co3+, Co2+, Cu2+, Mo2+, Fe3+, Fe2+, Cr3+, Ni2+, Ce3+ and Ce2+) promote the copolymerization of ethylenically unsaturated dicarboxylic acid monomers, especially maleic acid, and α,β-ethylenically unsaturated monomers having carboxyl or sulfonic acid groups, such as (meth)acrylic acid or 2-acrylamido-2-methyl propane sulfonic acid, whereby the amount of unreacted dicarboxylic acid monomer is significantly reduced. The metal ions are added to the monomer mixture. The resulting copolymer is not described as a superabsorbent polymer but is useful in antiscalants, dispersants, detergent additives, deflocculants etc.
Numerous other methods are known to reduce the amount of residual monomers in superabsorbent materials, for example, the use of sulfites, bisulfites, ammonia, amines, amino acids like cystein and lysine, sulfurous acid, phosphorous acid, pyrophosphorous acid, hypophosphorous acid, thiosulfuric acid, hydroxylamine or a salt thereof, and ascorbic acid (see U.S. Pat. Nos. 5,229,488, 5,866,678, 4,766,173, and 4,929,717).
The various methods taught in the prior art use additional non-reactive additives in addition to the reactive agents mentioned above and/or additional mixing measures to improve the distribution of the reactive agents. The non-reactive additives may be substances that can modify hydrogel particles, and may be not only surfactants but also, for example, organic solvents, mineral oils and very fine inorganic or organic particles like silicon dioxide, aluminum dioxide, zeolite or poly(methyl methacrylate) particles. The use of such additives complicates the manufacturing process, which then is more time-consuming and therefore less economical. In addition, the use of additives such as surfactants and very fine powders may adversely affect absorbence and liquid distribution.
It would be highly desirable to provide a novel method for the preparation of water-absorbent polymers having a low residual monomer level, which method would eliminate the above-mentioned disadvantages.